A passive optical network (PON) comprises an optical line terminal (OLT) connected to multiple optical network units (ONUs) in a point-to-multi-point network. New standards have been developed to define different types of PONs, each of which serves a different purpose. For example, the various PON types known in the related art include a Broadband PON (BPON), an Ethernet PON (EPON), a Gigabit PON (GPON), and others.
An exemplary diagram of a typical PON 100 is schematically shown in FIG. 1. The PON 100 includes N ONUs 120-1 through 120-N (collectively known as ONUs 120) coupled to an OLT 130 via a passive optical splitter 140. In a GPON, for example, traffic data transmission is achieved using GPON encapsulation method (GEM) encapsulation over two optical wavelengths, one for the downstream direction and another for the upstream direction. Thus, downstream transmission from the OLT 130 is broadcast to all the ONUs 120. Each ONU 120 filters its respective data according to pre-assigned labels (e.g., GEM port-IDs in a GPON). The splitter 140 is 1 to N splitter, i.e., capable of distributing traffic between a single OLT 130 and N ONUs 120.
In most PON architectures, the upstream transmission is shared between the ONUs 120 in a TDMA based access, controlled by the OLT 130. TDMA requires that the OLT first discovers the ONUs and measures their round-trip-time (RTT), before enabling coordinated access to the upstream link. For example, in a GPON network, during initial set-up an ONU 120 and the OLT 130 may be in one of the following operational states: serial number acquisition or ranging. In the serial-number acquisition state, the OLT 130 tries to detect the serial number of an ONU 120. If the OLT 130 and an ONU 120 have not completed the serial number acquisition stage, due to a low power signal, the ONU 120 independently changes its optical power output until a successful detection of the serial number. In the ranging state, the OLT 130 tries to determine the range between the terminal units (i.e., ONUs 120) to find out at least the round trip time (RTT) between OTL 130 and each of the ONUs 120. The RTT of each ONU 120 is necessary in order to coordinate a TDMA based access of all ONUs 120 to the shared upstream link.
During a normal operation mode the range between the OLT 130 to the ONUs 120 may change over time due to temperature changes on the fiber links (which results with varying signal propagation time on the fiber). The OLT 130 continuously measures the RTT and adjusts the TDMA scheme for each ONU accordingly.
In order to enable protection in PONs a redundant optical link and OLT are connected to a splitter. This type of a configuration is usually referred to as a duplex PON system. An example for such a system is a protection type B, defined in ITU-T standard G.984.1. An illustration of a duplex PON 200 is provided in FIG. 2. As can be noticed two OLTs 210-1 and 210-2 are respectively connected via optical fibers 220-1 and 220-2 to a splitter 230. The splitter 230 splits incoming traffic to N OUNs 240-1 and 240-N. That is, in this example, the splitter 230 is a 2 to N splitter.
In the duplex PON 200 one of the OLTs is set as active while the other as a standby. When there is a failure in the active OLT or its respective fiber a fail-over is performed and the operation is switched to the standby OLT and traffic is sent through its respective fiber. Typically, a synchronization link is established between the OLTs 210 to transfer database updates, PON status messages and switch-over trigger signals.
However, in order to ensure minimal service interruption due to a switch-over action, the standby OLT should perform at least the ranging process when turning into an active OLT. The lengths of the standby and active optical links are not the same. Performing such a process when establishing the network is an error prone approach as the “range” between the OLTs 210 and the terminal units (i.e., ONUs 240) may change over time and therefore there may be different RTT times and optical power levels for signals transmission power. Performing the ranging process for each ONU when switching-over is a time-consuming task and typically results with a long service interruption time, as the PON remains idle for duration that the ranging process takes place.
In addition, the topology of the network may change over time. For example, an ONU may be added or removed from the network. Thus, the standby OLT should maintain updated information relating to the status of the PON, as acquiring such information when switching-over is a time consuming process through which the PON remains idle.
A protection mechanism should maintain a fast and reliable communication channel between the two OLTs, whether the active and standby OLTs 210 are collocated on the same shelf, rack, or reside in geographically remote sites. In addition, a logic unit controlling the protection mechanism should be continuously updated with the status of both the fiber links connecting the standby and active OLTs 210 to the splitter 230. Since the standby OLT (e.g., OLT 210-1) cannot transmit data on its link (e.g., 220-1) while is in standby mode, thus, it is necessary to monitor the standby OLT link before a switch-over operation.
It would be therefore advantageous to provide an efficient protection mechanism for PONs.